1. Field of Invention
This invention pertains to electronic and coating applications. More precisely, this invention involves film-substrate interfaces used in electronic and coating industries. Specifically, this invention reveals a method of maintaining a constant depth during scratch testing and measuring the interfacial adhesion between substrates and films or coatings in micro/opto-electronic and protective/decorative coating applications.
2. Description of the Related Art
Quantitative tests for interfacial adhesion between films and substrates are of critical importance in determining the reliability of components in electronic and coating applications. However, many of the available quantitative tests are applicable only to a limited array of materials systems, or are experimentally complex.
Numerous tests have been proposed in the past to evaluate the quality of interfacial adhesion. Most of the proposed tests yield either qualitative information on interfacial bonding, or quantify the tensile strength of the interface. However, since the vast majority of interfacial failures in both electronic and structural applications occur in shear, it is crucial to quantify the shear strength of interfaces. So far, three different tests have been proposed to accomplish this.
The first test is the peel test reported by Spies in the Journal of Aircraft Engineering, 25 (1953) 64 and by Aravas, et al., in Material Science Engineering, A107 (1989) 159. In that test a fixture is attached to the film to peel the film from the substrate at a fixed angle, the force to do so is measured, and the work of adhesion calculated therefrom. However, the test is restricted to ductile films of greater than .about.100 .mu.m thickness which do not break or tear during peeling, and is not applicable to thin, relatively brittle coatings.
The second test is the indentation test reported by Marshall and Evans in the Journal of Applied Physics, 56 (1984) 2632; Rossington, et al., in the Journal of Applied Physics, 56 (1984) 2639; Engel in International Journal of Adhesion and Adhesives, 5 (1984) 455; and Lin, et at., in the Journal of Materials Research 5 (1990) 1110. In that test a loaded indentor is placed on the film attached to the substrate, and the load is incrementally increased until relative lateral displacement between the film and the substrate underneath the indentor causes debonding of the film from the substrate. The critical load for debonding is measured and converted into an interfacial shear strength or fracture toughness. However, that test is somewhat limited in scope, since it requires that the exact applied load at which the interface shears be determined. This is usually accomplished for transparent substrates by using a microscope located below the sample being indented to visually observe debonding, as reported by Lin, et at., in Journal of Materials Research, 5 (1990) 1110. The load for opaque substrates may be determined using acoustic emissions monitoring as reported by Sachsem, et al., in Proceeding MRS Symposium, 154 (1989) 293. However, since large acoustic signals are emitted during indentation due to a number of damage processes, detection of a signal from debonding of the film is usually quite difficult.
The third test which has been proposed to evaluate the quality of interfacial adhesion is a contemporary version of the scratch test, where the film-substrate pair is scratched at a constant speed under a progressively increasing vertical load, as reported by Valli, et al., in Journal of Vacuum Science Technology, A3 (1985) 2411; Julia-Schmutz, et al., in Surface and Coatings Technology, 48 (1991) 1; Wu in Journal of Materials Research, 6 (1991) 407; and Venkataraman, et al., in Thin Solid Films, 223 (1993) 269. In this test, the film eventually detaches from the substrate ahead of the indentor during scratching. The initial debonding event is detected via acoustic emission or from some characteristic of the load-time plot, and the interfacial strength is then determined from the vertical and horizonal loads corresponding to the initial debonding. However, because of the load-controlled nature of the test, significant junction growth (i.e., an increase in the actual area of contact between the microscopic asperities on the surfaces of the indentor and the sample) can occur during scratching, making the indentor ride up and down. This is a special problem for small film thicknesses, since the random variation in scratch depth can be significant in relation to the film thickness, leading to serious limitations associated with mathematical modeling of the process, and hence with the determination of interfacial strength. Additionally, as with the indentation test, there are experimental difficulties associated with the detection of initial interfacial debonding, making the test experimentally complicated and its results hard to interpret.